A new LESA life: screening food surfaces for pesticides by liquid extraction surface analysis

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  • Published: Aug 15, 2011
  • Author: Steve Down
  • Channels: Base Peak
thumbnail image: A new LESA life: screening food surfaces for pesticides by liquid extraction surface analysis

Pesticide traces on foodstuffs

Pesticides are still widely used in agriculture to aid the growth of fruits and vegetables, despite the steady growth of organic farming. Ideally, all traces should be removed by the time they come to market but this is not always the case. There are countless reports of contaminated produce but, fortunately, the level of contamination tends to be low in most cases.

In the USA, some foods are thought to be relatively highly contaminated, leading to the publication of the Dirty Dozen, which include peaches, apples, strawberries, spinach and lettuce. The cleanest produce is thought to include onions, cabbage, bananas and pineapples.

Unfortunately, there is no instinctive way of knowing that foodstuffs are pesticide-free, so batches have to be sampled and analysed regularly. Of the techniques currently accepted for food testing, many rely on homogenisation and extraction using a version of the QuEChERS method (Quick, Easy, Cheap, Effective, Rugged, and Safe).

However, there are alternatives which could prove to be quicker while retaining the sensitivity and reliability. One possibility has been explored by Jack Henion and Daniel Eikel from Advion Biosciences at Ithaca, New York, who exploited the speed of a new surface sampling technique linked to high resolution mass spectrometry.

LESA layout

The technique is called liquid extraction surface analysis (LESA) and it was reported in 2010 by scientists at Oak Ridge National Lab who designed it around the Nanomate chip-based infusion system for nano-electrospray mass spectrometry.

In a fully automated system, an autosampler picks up a blank pipette tip, draws up a small volume of extraction solvent and moves across to the sample. Here, it expels a droplet of solvent is that it comes into contact with the sample surface without fully leaving the pipette.

After a given time, which can be as short as a few seconds, the droplet is withdrawn and the pipette taken to a unique nozzle at the back of the nanospray tip where it dispenses the droplet for nano-electrospray analysis by infusion.

The extraction efficiency can be increased by dispensing and withdrawing the droplet several times at the same position before it is taken for analysis.

Rapid screening of fruit and vegetables

The system was illustrated by the extraction and analysis of several pesticides from apples, grapes and spinach. In the first instance, a mixture of five pesticides was hand sprayed from solution onto an apple and a slice was cut and placed in position in the autosampler.

The extract was analysed on an orbitrap mass spectrometer operating at a mass resolution of 100,000. The peak for the protonated molecule of malathion was measured with a mass accuracy of 2.7 ppm and no interfering peaks were co-extracted from the apple surface.

In a similar experiment, malathion was extracted and analysed by tandem mass spectrometry on a hybrid triple quadrupole-linear ion trap mass spectrometer. The full scan tandem mass spectrum of the protonated molecule matched that from a malathion standard.

These results illustrated the ease of screening, detection and characterisation of a single pesticide on the surface of a fruit, which had been sprayed at less than 400 ng/g. This is at least 20-fold lower than the US EPA tolerance limit, so the technique can operate well within statutory guidelines.

Next, simazine and sevine were sprayed onto an apple at 10 and 500 ng/g, respectively. These pesticides have the same nominal molecular mass but the exact masses of the protonated molecules are slightly different at m/z 202.0854 and 202.0864, respectively.

A mass resolution of 200,000 would be needed to resolve the pair but the presence of a chlorine atom in simazine gave a sufficiently strong isotopic peak at m/z 204.0825 to allow its confirmation. The same was not true for sevine, which had no convenient atoms to help in this way. However, tandem mass spectrometry analysis of the pair allowed their distinction due to the differences in the full scan product ion spectra.

Similar results were obtained for pesticides on grape skin and on spinach leaves, again at levels about 20-fold below the EPA tolerance limit. The control sample of spinach, which was purchased from a supermarket, was also analysed "neat" and was found to be contaminated with diazinon but at levels about 500-fold below that observed for signals from the artificially sprayed surface.

The method was not confined to solid fruit surfaces. Henion also illustrated the analysis of pesticides in a batch slurry of homogenised apples previously shown to contain pyrimethanil. A sample of the skin/pulp homogenate was air-dried for extraction and analysis on both mass spectrometers.

The protonated molecule and the tandem mass spectrum clearly confirmed the presence of this pesticide which was present at 30-fold lower than the EPA tolerance limit.

At this stage, the system is designed for detection and confirmation rather than quantitation but preliminary steps have been taken to investigate the automated loading of LESA samples into an injection loop for HPLC/MS experiments. In the meantime, Henion recommended QuEChERS extraction followed by GC/MS or LC/MS with stable isotope-labelled standards for quantitation.

LESA could be used to screen for any pesticide using a library holding the exact masses of all known pesticides. Its simplicity, speed, automated capabilities and simple sample preparation procedures make it an attractive option for checking the surfaces of fruits and vegetables for pesticides and ensuring consumer safety.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Pesticides on food surfaces at levels 20-fold lower than current EPA tolerance levels have been detected by mass spectrometry following automated sampling by liquid extraction surface analysis, providing a rapid screening procedure for food supplies

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